Satellite for increasing the utility of satellite communication systems

ABSTRACT

An existing TDRSS satellite communication system is incorporated, together with low-power ground-based remote transceivers of special design, and additional beamforming and steering elements at the ground terminals, to make possible digital communication between low power field transceivers and satellite ground terminals. The satellite communication system transmits to its ground terminals a composite signal, comprising amplified, phase-coherent signals received by an array of broad-coverage antennas on the satellite. The field transceiver transmits a pseudonoise coded signal spread across all or a portion of the satellite&#39;s receive bandwidth. At the ground terminals, the downlinked composite signal is processed by a beamformer to define a narrow, high-gain beam between the satellite and low-power transceiver. Signal processing gain and beamformer gain in combination serve to elevate the received, demodulated signals well above the noise level at the receiver. Through this invention, a large multiplicity of non-interfering reverse-link (remote-to-central) communication channels may be supported by a host satellite communication system of the nature described. By the introduction of forward link signal channels into the satellite ground terminal, and scheduling of forward link transmissions to the set of low-power remote transceivers along with normal transmit activities, highly useful two-way communication can be extended to a class of users not initially served by the satellite communication system.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.08/154,410 filed Nov. 19, 1993 now U.S. Pat. No. 5,572,216 for SYSTEMFOR INCREASING THE UTILITY OF SATELLITE COMMUNICATION SYSTEMS.

BACKGROUND OF THE INVENTION

There is an immediate and growing need for satellite-based, globalcommunications on-demand, between a hand-held user transceiver and acentral gateway or hub. This need, which applies to both the U.S.government and non-government sectors, includes 24 hour-a-day randomaccess communications support for emergency indication, small sensors,law enforcement, and many other "remote" user scenarios. The emphasis ison the ability of the satellite system to accommodate transmissions fromhand-held units at anytime and anywhere; communications back to the usermust also be supportable, but typically in reaction to a transmissionfrom the user. Such hand-held, random-access communication scenarioscannot be accommodated by existing commercial satellite communicationsystems, but, according to this invention, can be readily supported by anovel and unique utilization of NASA's Tracking and Data Relay SatelliteSystem (TDRSS) or a similar satellite system. The associated TDRSSsupport, coupled with the transceiver technology and implementation, arethe subjects of this invention.

Most communication satellites operate at geosynchronous altitude, analtitude of about 22,000 miles, at which point the earth's disk appearsapproximately 20 degrees across. These satellite communication systemshave traditionally utilized broad-coverage antennas to concurrentlyreceive signals from, and transmit signals to, regional ornear-hemisphere areas, while remaining over a fixed spot on the earth'sequator. The broad antenna beam, at typical frequencies (e.g.,microwave), corresponds to a small-area transmit-receive antenna. This,in turn, limits the electromagnetic power the antenna can intercept. Theresult is that, for acceptable communication quality, users on theground¹ must have relatively large antennas and/or transmit many wattsof power; this, in turn, typically leads to transceivers that cannot behand-held and, further, precludes efficient battery-powered operation.

Typical satellite transponders (that is, the on-board equipment forrelaying signals within a given frequency bandwidth) are in essenceamplifier-frequency-shifters which can accept signals from anyuser-transmitter on the ground operating within the band covered,amplify those signals, shift their frequency and retransmit them throughanother antenna to a central gateway. Since the signals are notdemodulated or signal-processed on-board the satellite, there is noprocessing gain to compensate for low signal power.

Special purpose communication satellites (e.g., for the Department ofDefense) have been built for a variety of purposes. With a largerantenna on the satellite, it is possible to communicate with a user onthe ground having a correspondingly smaller antenna and/or transmitterpower. In this case, however, the beamwidth of the satellite's antennais reduced, thereby requiring the location of the ground user to beknown, and the satellite's antenna tracked to that location. Were theantenna mechanically tracked by rotating itself or the entire satellite,that would use propellant at an unacceptable rate; what's more, it couldserve concurrently only users in a small area of the earth. Electronicantenna steering provides a highly attractive alternative thateliminates the disadvantages of mechanical steering while simultaneouslyproviding the ability to focus on (or null) many regions concurrentlywith high (or low, for nulling) gain; electronic steering can also beaccomplished much more rapidly than mechanical steering, again withoutany incurred mechanical satellite motion. Electronic steering is moreexpensive than traditional non-steerable antennas, and have heretoforeappeared mainly on military satellites. Furthermore, even on suchmilitary satellites, the number of simultaneous receive beams that canbe formed, and their operational flexibility, has been limited by thespecific on-board beamforming capability employed. In this regard, theelectronic beamforming capability used by the TDRSS is especiallyunique.

To satisfy its needs for global communications with low earth-orbitingspacecraft, NASA has developed the Tracking and Data Relay SatelliteSystems (TDRSS), which includes geosynchronous satellites that are ableto electronically steer an on-board phased-array antenna. This phasedarray views the entire earth's disk, but can form many simultaneousbeams to support reception of many independent user transmissions; eachsuch beam has a beamwidth considerably narrower than the earth's diskand thus also provides considerably higher gain than an earth coveragebeam. Furthermore, this same phased-array can form a single narrow beamat a time to provide high power transmissions back to the user; thisbeam can be independent of, or directly related to, any of the manysimultaneous receive beams. As such, both the receive (inbound or returnlink) and transmit (outbound or forward link) beams are sufficientlypowerful to accommodate low-power; hand-held user transceivers; not onlyis this operationally attractive to the user but it also provides theadded benefit of extended battery lifetime and reduced exposure to RFemissions.

Electronic beam steering requires that signals from a number of separateantenna elements, most commonly arranged in a planar area, bephase-shifted by amounts depending on the distance of the element fromthe center of the array and the direction in which the beam is to form.Whether the application is radar or communications, such antennastypically have their beamforming accomplished at the antenna. In thisregard the TDRSS is unique, in that the inbound (i.e., return link)beamforming is performed on the ground. Specifically, the TDRSStransmits the signal, received by each on-board antenna element,separately to the ground station in a composite, frequency-multiplexedsignal. Since the coverage of each element of the TDRSS is more than theangle of the earth's disk, the combination of signals sent to the groundcan be combined on the ground to "form" a much narrower beam and todirect it, free of any mechanical inertia. This has several advantagesrelative to conventional approaches of beamforming at the antenna.First, a beamformer on the ground can be replaced if a failure occurs.Second, the number of independent beamformers can be much greater on theground than can be possibly placed on-board a satellite. Third, thenumber of independent beamformers can be expanded and independentlyallocated to independent users, if needed, after the satellite is inorbit, and one or more receivers can be attached to each beamformer.Finally, the beamforming algorithms can evolve and improve withtechnology, if the beamforming is accomplished on the ground and caninclude split beams, nulling, and other forms of enhanced antennapointing. Clearly, all of these advantages of ground-based beamformingyield a greatly increased satellite "return on investment".

Many global voice and data communication needs, both government andnon-government, remain unmet through application of conventionalcommunication satellites. More particularly, few (if any) satellitesensor or communication systems can communicate flexibly (e.g., demandor random access) and successfully with small, low-powered, hand-held,low-data-rate ground-based user transceivers, or with remote instrumentsor controllers not equipped with large antennas or, equivalently, withhigh output power.

In addition to its services using electronically steered antennae, TDRSSalso provides additional services via mechanically steered antennae.These single access (SA) services, at S, Ku and future Ka band, providelinks with attractive bandwidth and link closure properties. Use ofthese services require scheduling; however, normal mission users do notrequire all of the schedulable service and spare satellites on orbithave no scheduled use. As such, there is a strong potential to useexcess capacity within the SA services to also accommodate new groundusers. For example, field users and/or users on the ground terminal sidecould store information over, for example, a period of hours. At ascheduled time, when no normal users required services, all or part ofthe stored information could be forwarded (i.e. transmitted) throughTDRSS via one of the available services. Hence, this store-and-forwardtechnique could be used to pass data to/from the field during intervalsfor which TDRSS had no other service requirement. As another example,since each TDRSS single access antennae is multi-band (S, Ku now; S, Ku,Ka in the future), only one frequency is typically used at a time. Assuch, if an independent user is within the beamwidth of an alreadyscheduled single-access service, the independent user can take advantageof the "free" frequency to obtain single access service without the needfor separately scheduled single access time.

Given the wide range of services and service types, there are numerouspotential ground-based uses of TDRSS, even outside the area of hand-heldtransceivers, which are worthy of consideration. For example:

1. The ability to support high bandwidth broadcast and low rate requestchannels in a single satellite makes TDRSS a candidate for asymmetricbroadcast services (e.g., the Global Broadcast System) for providingwireless Internet access, etc.

2. Packet data transfer via TDRSS, with short (e.g. <1 second) datapackets, is possible using modified receiver technology to achieve rapidsignal acquisition. Current TDRSS operations provide continuous serviceover contact intervals which may range from seconds to minutes induration.

3. Field equipment may be tailored to mesh with services. For instance,low/high gain antennae, wide/narrow beam width antennae, low/high poweramplifier, etc. can be employed to satisfy specific ground applications.

4. TDRSS is a "bent pipe" and data flow through it has no formalrequirements in terms of content, security/encryption, coding, etc. Inessence, all data may be passed over TDRSS given the RF signalcharacteristics are acceptable.

The object of this invention is to provide an improved global satellitecommunication system that uniquely applies the TDRSS without impact toits prime mission of supporting low-earth-orbiting science spacecraft;or another suitably implemented satellite system. This inventionencompasses both the satellite system concept including the groundterminal, and the ground-based transceiver design and implementationrequired for successful system operation.

DESCRIPTION OF THE DRAWINGS:

The above and other objects, advantages and features of the inventionwill become more apparent when considered with the followingspecification and accompanying drawings wherein:

FIG. 1 is a diagrammatic overview of a satellite communication systemincorporating the invention,

FIG. 2 is a more detailed diagrammatic view with a block showing keysystem characteristics,

FIG. 3 is a block diagram of the multiple access (MA) beam formingmulti-channel receiver configurations incorporating the invention,

FIG. 4 is a generalized block diagram of a transceiver incorporating theinvention,

FIG. 5 is a more detailed block diagram of a compact/low power TDRSSforward link receiver incorporating the invention,

FIG. 6 is a block diagram of a remote transceiver incorporating theinvention and illustrating available expansion options,

FIG. 7 is a block diagram of the S-band MA return link,

FIG. 8 is a block diagram of the S-band MA forward link, and

FIG. 9 is a block diagram showing the tracking data rely satellite usedin a Global Broadcast (or similar) System.

DETAILED DESCRIPTION OF THE INVENTION:

The heart of this invention is the employment of the existing TDRSSsystem (see FIG. 1), or others similar to it, to carry on communicationbetween small, low-power field stations and a ground terminal suitablefor receiving the composite downlink signal and/or the SA downlinksignal transmitted from a TDRSS satellite. The elements shown as "TDRSSGround Terminal" (GT) in FIG. 1 could be either new ground stationsreceiving the TDRSS satellite signal and providing receive service forlow-power field stations, or existing TDRSS ground terminals supplyingtheir composite downlink signals to beam former and receiver circuits(FIGS. 2 and 3) supporting each concurrent added field channel. In FIG.2, for simplicity, only the two frontside constellation nodes 174'W and41'W are shown.

The system of FIG. 1 provides:

1) Global coverage with inclusion of backside node;

2) TDRSS provides desired support with no satellite modifications;

3) Non-NASA traffic operates CDMA (or other non-interfering waveform)via TDRSS within normally used spectrum without interference (i.e., nodedicated or new spectrum for MA and some SA service);

4) Non-NASA user has access to multiple access (MA) return link (i.e.,inbound from field-to-TSRSS) at any time, anywhere without scheduling;

5) Multiple Access (MA) forward link (i.e. outbound from TDRSS-to-field)and single access (SA) forward/return links involve coordination withNASA.

In the block diagram of FIG. 3, RF frontend, multiple access elementsseparators and beamformers are contained in block 20 with the IF outputof each beam, Beam 1, Beam 2 . . . Beam N being supplied to theirrespective multi-channel receiver for beamformers 21-1, 21-2 . . . 21-N,each of which has its end user channel processors 22-1, 22-2 . . . 22-Kand each of which has a CCD correlator 23, which may be of the typeshown in Weinberg et al. U.S. Pat. No. 5,126,682, the output of which issupplied to A/D converter 24 which, in turn, supplies its digital outputto digital signal processor 25 and thence to the user. Processor 25provides a PN code/sample control feedback loop 26 to correlator 23.Note that the existing ground terminal receiver equipment differs fromthis embodiment but is also applicable as are other receiver types.

A general block diagram of one embodiment of the low-power, hand-heldfield transceiver described above is shown in FIG. 4. Preferably, itwill be battery powered and, in one embodiment, limited to communicationat data rates of, say, 2400 bits per second or less, using 1-2 watts oftransmitted RF power. This is sufficient to transmit coded voice, andgenerally speaking, more than sufficient to transmit computer generatedsignals or output from monitoring or alerting devices.

A small S-band patch antenna 30 is connected to diplexer 31 whichsupplies signals to the TDRSS forward link receiver 32, and receivessignals for transmission from TDRSS return link transmitter circuit 33,both receiver 32 and transmitter 33 are managed by digital dataprocessor 34. Most of the transceiver can be implemented in a single,all digital application specific integrated circuit. The transceiver canhave separate transmit and receive antennas, in which case, diplexer 31is not required. The antenna(e) for the receiver and transmitter can beselected to provide desired gain and pattern characteristics. Higher (orlower) data rates may be accommodated by proper selection of theantennae), power amplifier, and low noise amplification. A miniature(e.g. 5-channel) GPS receiver 36 coupled to a an L-band antenna 37supplies GPS position information to the user. It will be appreciatedthat GPS Message Data (ephemeris, almanac, etc.) can be supplied viareceiver channel 32 to aid in acquisition of the GPS satellite signals.

In the battery powered transceiver depicted in FIG. 4, in the preferredembodiment, the transmitter 33 and receiver 32, operate non-coherentlywith one another, that is, without the requirement that transmitted andreceived signals bear a fixed electrical frequency and phaserelationship associated with the TDRSS coherent turn-around mode. Thisfeature arises from the desire to keep the transceiver simple and robustand of low power consumption. It places the onus for frequency trackingon the ground terminal beamformer, receiver and transmitter. However,since this particular low-power transceiver is intended for operation onland, sea, or air rather than space, the added tracking effortassociated with large doppler shifts is not imposed on the groundterminal equipment. The embodiment shown in FIG. 4 incorporates a GPSreceiver 36 which accomplishes position location, both for its user andfor use by the TDRSS equipment that is assigned to it so thatpointing/nulling accuracy may be enhanced. Moreover, the positionlocation and other data can be caused to be transmitted automatically orby a triggering signal received by the transceiver.

FIG. 5 is a more detailed embodiment of a compact low power TDRSSforward link receiver wherein the RF input 40 is amplified by a lownoise amplifier 41, and the output down-converted by open loop RF to IFdown-converter 42. The output is sampled at a 24 mHz rate by sampler 43driven by clock 44. Sampled IF signals are sign detected in detector 45and supplied to a digital correlator (pseudonoise matched filter) 46(which is, for instance, a 128-256 stage correlator). Digital signalprocessor 47 receives the correlated signals from correlator 46 andforwards the signal to the data end user and also provides a PNcode/latch control signal to correlator 46. The components in the dashedrectangle DR can be implemented by specific circuits or by applicationspecific integrated circuit (ASIC) chips. The receiver design concept isshown in FIG. 5, and reflects the following:

a. PN code matched filter (PNMF) correlator for rapid PN acquisition.

b. IF sampled operation of the PNMF to simultaneously permit PNdespreading and demodulation to baseband, while also leading to areduction in the required number of components.

c. Open-loop downconversion to IF, and open-loop sampling at the PNMFinput, to enable simplified frequency synthesis and reduced powerconsumption.

d. All acquisition and tracking processing, following the PNMF, via acombination of ASIC(s) and a low complexity microprocessor.

e. A nominal number of programmable, low data rate settings (e.g., 600bps. 1.2 kbps, and 2.4 kbps), that are amenable to low complexityprocessing.

f. Preferably, power consumption is kept below 3 watts.

Alterations to the FIG. 5 embodiment can be made to accommodate otherspread and non-spread waveform types which are deemed acceptable for useon TDRSS.

The robust nature of this system facilitates its use in a wide range ofapplications. FIG. 6 illustrates some of the accessories that may beinterfaced with the field transceiver. Under this approach a commonfield transceiver 50 (or class of such transceivers) can be developed tosatisfy numerous types of applications. It is also possible to integratethe accessories within the transceiver to satisfy special applicationsthat require size, weight and cost reduction.

As illustrated, position systems, radar 51, inertial guidance unit 52,dead reckoning 53, loran 54, and GPS 55 can have their information usedby the transceiver application and/or communicated to points remote fromthe field transceiver. Remote control element 56, alarm 57 and othersensors 58 can be coupled to the transceiver 50 for use by thetransceiver application and/or transmission of data sensed therein to aremote location. Voice channel 60 for voice communication, through thetransceiver, with a remote location transceiver control monitor unit 61for use by the transceiver application and/or transceiver operation andstatus. The transceiver can also be coupled to a cellular network viacontrol modem 62, cellular phone handset 63 and transceiver 64. Variouscomputer-type interfaces 65 can be coupled to the transceiver 50 as wellas a printer 66. Conventional Telco circuits 67, computer 68, modem 69,recorder 70, encryption/decryption device 71, and video display andcameras 72 may be incorporated in the system. If vehicle mounted thetransceiver 50 can be powered by electrical power 73 from the hostvehicle. Other power sources are also possible.

FIG. 7 is a block diagram of an S-band return link wherein a fieldtransmitter FT transmits its signal on the S-band which is frequencyshifted by TDRSS satellite to a Ku-band signal and transmitted to aground terminal for processing via RF front end RFE, an elementseparator ES and the elements coupled to an array of beam formers BF1 .. . BFN and the individual signals of each beam former being supplied toreceivers RC1 . . . RC-M.

FIG. 8 is a block diagram of the S-band multiple access forward linkillustrating a ground terminal having a command transmitter CT havinginputted thereto the MA transmit antenna-beam weight for transmission tothe satellite via the Ku-band signal and data for upconversion to aKu-band signal for transmission via the satellite TDRSS on S-band linkto a field receiver.

FIG. 9 illustrates use of the invention for a global broadcast in whicha data or information request issued by computer CD is forwarded on alow data rate channel which includes S-band request channel to satelliteTDRSS. The data request is forwarded by satellite TDRSS on the Ku-bandsignal to a ground terminal which stimulates a response which isforwarded, in this case, on the Ku-band uplink. The satellite then beamsthe response to a broadband high data rate receiver which is coupled tocomputer CD.

It should be understood that the functions of the field transceiver arenot limited to those described specifically, but may include signallingfrom remote radar, navigation, sensors and controls, encoded voicetransmissions, emergency alarms and other applications.

The invention provides unique satellite communication applications viaunique utilization of the multi-access (MA) (and in some cases thesingle-access (SA)) capability of NASA's Tracking and Data RelaySatellite System (TDRSS) or similar satellite communication systems.This capability of the invention is termed "efficient satellitecommunication (SATCOM) provider" or "ESP". The invention, in itspreferred embodiment, would:

1. "Overlay" ESP signals on existing TDRSS signals in the same MAbandwidth or time share the bandwidth for MA and/or services.

2. Largely be independent of, or of manageable impact on, normal of NASATDRSS operations.

3. Be mutually non-interfering.

4. Permit global random access communications over the MA field-to-hublink.

5. Permit dramatic reductions in field-user effective isotropic radiatedpower (EIRP) relative to existing geostationary satellites, such asINMARSAT.

6. Apply new, advanced signal processing to enable rapid acquisition(for reduced overhead) high-performance, very low power consumption, andhand-held field unit implementation.

Unique features of the preferred embodiment of the invention include thefollowing:

1. The return (inbound) communications link (from field-to-hub groundterminal) capability is achieved via unique utilization of the TDRSS MAground-based phase array beamforming capability. In particular, theinvention involves augmenting the existing NASA ground beamformers withseparate beamformers, of sufficient quantity and independent of NASAoperations. This ground beamforming approach, which is not available viaany other satellite system in the world, is the key to permittingindependent operations and permitting continuous global coverage. Anexample is use of this application as a random access channels forpassing information requests in support of a Global Broadcast System.Operations of this nature are unique to this invention.

2. By employing a sufficient number of independent ground beamformers,per TDRSS satellite, the ESP inbound link is continuously available toground (including air and sea) users in the field, at all globallocations except at extreme latitudes. Specifically, this inbound linksupports random access inbound transmissions at any time and anywhere,and is totally independent of normal NASA operations. In addition,multiple receivers/demodulators may be attached to each beamformer tohandle multiple independent users within a beam.

3. The ESP inbound beamforming capability also provides the unique,simultaneous capability of global coverage and very low field-userEffective Isotropic Radiated Power (EIRP); such a capability is notavailable via any other existing or planned satellite system. Inparticular, the global coverage is provided by the 26 degree beamwidthof each MA element on-board each TDRSS; this broad beamwidth is typicalof "earth coverage" geostationary satellite antennas, such as INMARSAT.What is unique here is that the ground beamforming coherently combinesthe signals from 30 TDRSS antenna elements to yield approximately 14 dBG/T gain, thereby effectively reducing the user's required EIRP by 14 dBrelative to conventional satellites. This can provide a battery lifetimefor TDRSS user equipment that is substantially longer than a comparabledesign for an INMARSAT-type system. Furthermore, because the beamformingis performed on the ground, numerous independent beams can be formed onthe ground, thereby yielding the desired, simultaneous global coverageand high G/T.

4. The invention's inbound communications can support many ESP userssimultaneously without mutual interference. In addition, ESP operationcan also occur simultaneously with ongoing, normal TDRSS operations viathe same TDRSS MA antenna over the same operating bandwidth. In otherwords, ESP MA communications can "overlay" on TDRSS communicationswithout mutual interference. This is accomplished by a combination ofdirect sequence code division multiple access (DS-CDMA) and the groundbeamforming capability. Specifically, the ground beamforming capabilityprovides spatial discrimination via its above-described ability to formmany spot beams. In addition, ESP and TDRS users employ CDMA withdistinct PN codes to generate additional levels of discrimination viaprocessing gains approaching 30 dB for data rates of particular interest(preferably on the order of a few kilobits per second (kbps)). Higherdata rates (e.g., up to 150 kbps) can also be used with CDMA but withcorrespondingly reduced processing gain. Furthermore, within this CDMAframework, the above-described ground beamforming reduces the fielduser's EIRP well below the CDMA noise floor, thereby further reducinginterference levels. In fact, communication link analysis indicates that40-50 simultaneous CDMA users can transmit within a ground-formedbeamwidth and increase the effective noise floor by no more than 0.25dB. Note that other forms of spread spectrum signalling is possible ifdeemed acceptable to TDRSS.

5. The above robustness to mutual interference, which is based on thecombination of spatial and signal diversity, can be further enhanced bysuitable use of packet communications that introduces the additionaldimension of time diversity. Precise timing can be accomplished byintegrating a GPS receiver with the field unit, using the forward linkfor loop closure, or other means. The use of rapid acquisitiontechnology in conjunction with spread spectrum signalling offers themeans to resolve packet collisions which may occur if a precise timingbase is not used.

6. An important element of the ESP inbound ground receiver is thedemodulator/detector that interfaces with each beamformer. This elementcan incorporate advanced charge coupled device (CCD) and DSP technologyto permit extremely rapid acquisition, high performancetracking/detection, PN code and data rate programmability, and overalllow implementation loss. This element, and the associated signalprocessing algorithms, are disclosed in Weinberg et al. U.S. Pat. No.5,126,682, assigned to the assignee hereof.

7. The nominal ESP mode of operations indicated above is inboundoperations, which can be totally independent of TDRSS operations. Wheninbound operations must be supplemented by outbound (hub-to-field)communications--e.g., to support certain emergencies, such as search andrescue--coordination with NASA must take place in order to scheduletransmissions to the field via the TDRSS MA phased array or S/K-band SAservices. Alternatively, outbound communications can take place via anyother satellite system (e.g, INMARSAT, DirecPC, GBS), if desired. TDRSSMA service, however, has a distinct advantage to the field user incertain applications given the high EIRP it provides (≧34 dBW), relativeto Satellites such as INMARSAT.

8. To support outbound, spread-spectrum communications via TDRSS, aunique receiver concept is employed that permits dramatic reductions inpower consumption, while simultaneously permitting PN data rateprogrammability, rapid acquisition and high performancetracking/detection. In a preferred embodiment, the receiver employs aunique combination of open-loop IF sampling, sign-bit detection (no A/Dconversion), PN code matched filtering (PNMF) in a long correlator thatdoes not require any multiplications, and all carrier, PN, and symbolsynchronization in post-detection software. The nature of the receiverprocessor readily lends itself to ASIC miniaturization, and hence tohand-held implementation. An advanced CCD correlator implementation mayalso be used in place of the single bit detector/PNMF.

9. In a preferred embodiment, the receiver field unit could alsoincorporate a commercially available miniature GPS receiver. The GPSreceiver's position output may be relayed back to the hub via the fieldunit transmitter. Data collected by the field unit's receiver may beused to provide the GPS receiver with almanac, ephemeris, differentialGPS and configuration information.

10. The system is a digital communication system that affords tremendousflexibility. The hub and field transceiver may be fitted with a widerange of industrial and custom interfaces of data, voice, imaging,multimedia and other types of information transfer. For example: RS-232,RS-422, IEEE-488, Local Area Networks, telephone and fiber opticinterfaces are all feasible as well as all types of protocols (e.g.,TCP-IP).

11. This system is well suited for use in mobile applications requiringnavigation and tracking. GPS, Loran, dead reckoning, beacon, inertialnavigation and radar systems may be used. In addition, the fieldtransceiver may be integrated with the electronic systems of cars,boats, ships, and aircraft to access health and status information aboutthe vehicle, to support/direct its operation, and to operate as part ofthe information system employed by operators and passengers.

12. A wide range of alarms and sensors may be interfaced with the fieldand hub communication systems. The system can convey command and statusinformation. These include, but are not limited to, theft/break-inalarms, environmental monitors, emergency/distress indicators, medicalmonitors and personal security systems.

13. This system is well suited for fleet management, collisionavoidance, pager, packet, data transfer, digital voice, data, modem/fax,imaging, multimedia, navigation/tracking (e.g. personal, vehicular andwildlife) applications and global connectivity of classrooms, hospitals,etc.to support international/interactive education, treatment, etc.

14. The hub and field communication units may be interfaced withtelephones, mobile telephones, printers, recorders, encryptor/decryptor,modems, computers, navigation/tracking equipment, vocoders andsensors/alarms to support a wide range of applications. The TDRSS actsas a bent pipe system and permits use of all information security meansemployable via one- and two-way SATCOM channels.

15. Transmit power levels from the field unit for a select data rate issignificantly lower with TDRSS than with other existing geosynchronoussystems. This aspect results in more efficient use of battery power andreduced exposure to RF emissions.

16. The beamforming capability of the MA forward and return links permitselective beamforming. Said beamforming may include beamsplitting,selective nulling, etc.

17. The single access services of TDRSS may also be employed either inconjunction with or separate from the MA services to support globalcommunications. Excess SA service may be used without impact to normalNASA missions by scheduling unused time or by other forms of signaloverlays acceptable to NASA. Unused time may be used by entitiesoperating in a store-and-forward mode.

As noted above, the most significant feature of the MA system is itsground-based antenna beamforming capability that simultaneously yieldsearth coverage, and up to 14 dB of G/T enhancement relative toconventional earth coverage geostationary satellite antennas. The CDMAnature of the MA system also permits efficient spectrum utilization, andprecludes interference impacts on normal user spacecraft MA operationseven if a large number (e.g. 50) of low-power, non-NASA users aresimultaneously transmitting.

As shown above, the field transceiver integrates a TDRSStransmitter/receiver for communications, and an optional GPS receiverthat provides accurate estimate of field user position and time. Thenoncoherent nature of the TDRSS transmitter/receiver, coupled with theapplication of digital Application Specific Integrated Circuits (ASIC's)yields compactness and an anticipated low receiver power consumption ofless than about 3 watts. Furthermore, the MA G/T enhancement permits upto 1.2-2.4 kbps return link support via .sup.˜ 1 watt of RF transmitteroutput power and a near-omni antenna. In addition, the GPS receiver canbe an off-the-shelf item that is miniature in size (.sup.˜ 2" square)and consumes less than 1 watt of power.

Communications requirements of various U.S. Government agencies, as wellas non-government groups, is increasingly emphasizing the need forsatellite-based, global communications between a hand-held "field"transceiver and a central government hub. These requirements are alsoemphasizing the need for instantaneous communications on demand, and theutilization of spread-spectrum signalling forlow-probability-of-intercept/low-probability-of-detection (LPI/LPD).Several examples are:

1. DoD Combat Survivor Emergency Location (CSEL)--a global search andrescue capability for downed aircraft pilots, that requires theinstantaneous availability of a communications link from pilot-to-hub,followed by a rapid response from hub-to-pilot.

2. Special operations of the Department of Justice.

3. Portable, global communications by the Department of Transportation.

4. Department of Interior--monitoring and rescue of researchers/hikersin remote areas.

5. NOAA--science data collection from distributed science stations andassociated global interconnectivity of student centers.

The CSEL Program has received particularly significant attention overthe past few years, and has led to extensive DoD studies and assessmentson potential modifications to DSCS and GPS satellites needed toaccommodate near-instantaneous, global communications. Interestingly,these DoD efforts have attracted the interests of diverse U.S. agencies,such as Justice, Energy and Agriculture, all of which are intenselyinterested in global communications on demand via hand-held or othertypes of miniaturized transceivers.

It is within this framework that the TDRSS is emerging as a truly uniquenational asset, with features and capabilities not available in anyother U.S. or foreign satellite system, commercial or military.Specifically, application of the existing TDRSS Multi-Access (MA) systemunder the invention can provide the global communications on demand,described above, via a compact, very-low-power transceiver. Such anMA/hand-held transceiver application, would cost-effectively offer abroad range of U.S. government users--and non-government users--uniqueand important services not currently available, without impacting thefundamental TDRSS mission of supporting low orbiting NASA spacecraft.This MA application could also offer a platform in the sky" fortechnology development towards enhancing U.S. internationalcompetitiveness. Furthermore, because the existing constellation ofTDRSS satellites are applicable, these new services can start becomingavailable within the next few years, based on ground system upgradesonly--again a feature that cannot be provided by any other satellitesystem.

Successful ESP operations depends on the continuous availability oftwo-way satellite communications, with particular emphasis on theability of a compact field unit to transmit to the hub on-demand--i.e.,a return link at any time from virtually anywhere on earth. Theuniqueness of TDRSS MA utilization is based on this fundamentalrequirement, coupled with the diversity of several other requirementsthat the ESP capability should, preferably, satisfy:

1. Each satellite must provide continuous return link earth coverage,without service scheduling, while simultaneously providing a high G/T.This fundamental feature clearly includes the conflicting requirementsof broad antenna coverage and high antenna gain. TDRSS is the onlyexisting satellite system that can satisfy these conflictingrequirements because of its 30 element MA phased array antenna, coupledwith its unique return link ground beamforming capability. Each elementof the satellite array provides full earth coverage, while the groundbeamforming can provide a G/T enhancement on the order of 14 dB. Thus,since any desired number of simultaneous beams can be formed on theground, given a sufficient number of ground beamformers, the abilityexists for TDRSS to uniquely provide continuous global coverage, whilesimultaneously providing the field transmitter a G/T advantage of morethan 10 dB relative to a conventional earth coverage satellite antenna.As addressed further below, this introduces the potential for returnlink compressed voice (1.2-2.4 kbps) via a field transmitter power onthe order of 1 watt.

2. No interference impact on normal communication traffic over thesatellite. This is clearly a critical system feature. It is satisfied bythe TDRSS MA system, which was designed to intentionally support manysimultaneous Code Division Multiple Access (CDMA) signals without mutualinterference.

3. No dedicated satellite transponder bandwidth required. This is ahighly desirable feature, since some of the ESP services (e.g., searchand rescue; special operations communications) may be active only afraction of each day. TDRSS does not have to dedicate bandwidth to ESPbecause the MA system, which employs CDMA, always operates over its fullchannel bandwidth, and as noted above, many simultaneous users canoperate over this bandwidth without mutual interference.

4. LPI/LPD capability. The CDMA spread spectrum utilization of the MAsystem inherently provides LPI/LPD. In fact, for 2.4 kbps data orcompressed voice, the MA spread spectrum capability provides more than30 dB of processing gain. Furthermore, the high G/T provided by theground beamforming permits more than a 10 dB reduction in user EIRP,relative to a satellite with an earth coverage antenna; this EIRPreduction provides an additional degree of LPI/LPD not available viaother satellite systems.

5. Global coverage to ground users (except at extreme latitudes). TheTDRSS, with its existing three-slot constellation provides globalcoverage.

6. Availability of a forward link--from Hub to field. This is anessential ESP feature, with the nature of the forward link data afunction of the application (e.g., acknowledgement to the downed pilotof Hub receipt of emergency transmission). The TDRSS MA system certainlyhas this capability, but in contrast to the MA return link the forwardlink must be scheduled. Given the electronic steering of the MA system,however, antenna pointing in the desired direction can be done veryrapidly. In the event of an emergency, such an MA forward link cancertainly be established within a few minutes, and probably much less.Accurate position location information would be provided to TDRSS viaGPS-derived position data included in the MA return transmission. The SA(said K-band) could also be scheduled to provide forward link service ifneeded.

A global architectural overview is shown in FIG. 1, which illustratesthe three TDRSS nodes, its ground terminals (GT's), and the availabilityof GPS signals to globally distributed users. The following observationsapply:

1. Users are shown as people with hand-held transceivers. Moregenerally, as shown in FIG. 6 and discussed earlier, there is nothing topreclude the users from being instruments (e.g, science or other datacollection stations).

2. The user will typically initiate transmission in a purely randomaccess mode. Unique PN codes will be allocated to such users to ensurenon-interference with normal TDRSS users. The duration and duty cycle ofa transmission represents a subject for further study, but packettransmissions should maximize capacity and minimize the amount ofhardware required at the TDRSS GT. Many such transmission can occursimultaneously (e.g., 50) with negligible impact on normal TDRSS userspacecraft communications.

3. Each TDRSS GT is augmented with a sufficient number of beamformers topermit continuous, global coverage. The number of beamformers per GT iscurrently under investigation, but updated analysis to date indicatesthe need for 30-40 beamformers per GT. An option may be to apply fewer(e.g. 10-20) fixed beamformers with "defocused", lower-gain beams forinitial signal acquisition, and a few separate scanning beams that canprovide the maximum MA gain after signal acquisition. The crucial pointhere, however, is that the TDRSS MA system offers the capability andflexibility to support a variety of operational approaches. Multiplereceivers/demodulators may also be connected to each beamformer toincrease flexibility by accommodating many simultaneous users per beam.

4. One or more low-cost, multi-channel, rapid-acquisition receiver isconnected to each fixed beamformer; rapid acquisition is important inorder to minimize transmission overhead, thereby reducing transmissiontime and enhancing LPI/LPD performance. Each channel is matched to adistinct PN code (e.g., Code 1 for Department of Justice, Code 2 forSearch and Rescue, Code 3 for Department of Agriculture, etc.). Thelow-cost, rapid-acquisition attributes arise directly from proven NASAAdvanced Systems Program developments, coupled with the use of fixed,low-data rates. FIG. 3, discussed earlier, illustrates the GTbeamforming/receiver concept.

Upon signal acquisition and detection, the TDRSS GT forwards the data tothe appropriate end-user destination. End-user coordination with NASAtakes place as necessary to schedule the MA forward link (or possiblyother SA services) for transmission to the field user. For emergency orother critical scenarios the access to the MA forward link can beextremely rapid (e.g., a few minutes or less). Specific priorityarrangements would be included in the Memorandum of Agreement (MOA)between NASA or its representative and the respective user agency or itsrepresentative. It should also be noted that a current NASA AdvancedSystems Program study is investigating Demand Access utilization of theMA forward service, which would ensure rapid access without scheduling.

Because size and power consumption must be kept to a minimum, whilehigh-performance must still be achieved, the transceiver reflects thefollowing:

1. The TDRSS portion includes a transmitter and receiver that arenoncoherent. The absence of coherent turnaround operation greatlysimplifies frequency synthesis, which lends itself to simplicity,robustness and significant reductions in power consumption. For theground-based application of interest here, the absence of coherentturnaround is of little consequence, since it offers virtually notracking benefit; furthermore, position location and accurate time isaccomplished via GPS utilization.

2. The typical TDRSS S-band antenna is assumed here to be a small patchantenna with the approximate dimensions of 3"×3"×0.5". This antennaprovides a boresight gain of .sup.˜ 6.5 dB and a 3 dB beamwidth of 80degrees, thereby yielding attractive gain with little pointingcomplexity. Furthermore, the RF front end (transmit and receive) can beplaced very close to the antenna, thereby minimizing transmit losses andenabling an attractive receiver G/T (e.g., .sup.˜ -25 dB/° K.). Thisyields advantages to the current ground-based application which may notbe readily applicable to user spacecraft. Other antenna types, based onmission requirements, are also acceptable.

3. The high-performance of the TDRSS receiver (rapid acquisition, lowimplementation loss) arises from an innovative, all digital designconcept, that applies signal processing approaches that have beendeveloped, demonstrated, and continue to be refined, via the NASAAdvanced Systems Program. In addition, the design relies on extensiveuse of Application Specific Integrated circuits (ASIC's) to enabledramatic reductions in size and power consumption. Furthermore, thedesign concept takes advantage of the higher G/T as discussed earlier.

4. The TDRS transmitter outputs a PN coded signal using a Mode 2(noncoherent) TDRSS PN code. The output RF power is on the order of 1watt which, based on link budget analysis, yields an in-bound link datarate 1.2-2.4 kbps. This is a truly profound capability for a hand-heldunit transmitting to a geostationary satellite, and clearly reflects thebenefit of MA ground-beamforming.

5. The GPS receiver is a self-contained, miniature card residing in thetransceiver, that processes the GPS C/A codes and yields positionaccuracy of 1 km (3σ) or better. It should be emphasized that 5 to12-channel GPS receivers are available now, with dimensions no greaterthan 2"×3", and power consumption less than 1 watt. This thus representsa non-developmental item that is amenable to purchase and directincorporation into the proposed transceiver.

The invention provides an innovative global satellite communicationsconcept--that takes advantage of the unique properties of the TDRSS MAsystem to enable global, random access communications via compact,low-power field transceivers. The most significant feature of the MAsystem is its ground-based antenna beamforming capability thatsimultaneously yields global coverage and up to 14 dB of G/T enhancementrelative to conventional earth coverage geostationary satelliteantennas.

The CDMA nature of the MA system also permits efficient spectrumutilization, and precludes interference impacts on normal userspacecraft MA operations even if a large number (e.g., 50) of low-powernon-NASA users are simultaneously transmitting. This invention has alsopresented an operations-concept overview and a description of thecritical field transceiver. In particular, it was shown that the fieldtransceiver includes a TDRSS transmitter/receiver for communications,and a GPS receiver that provides accurate field user position. Thenoncoherent nature of the TDRSS transmitter/receiver, coupled with theapplication of digital ASIC's and NASA Advanced Systems Technologyinsertion, yields compactness and an anticipated receiver powerconsumption of less than 3 watts. Furthermore, the MA G/T enhancementpermits 1.2-2.4 kbps in-bound link support via .sup.˜ 1 watt of RFtransmitter output power. In addition, the GPS receiver is anoff-the-shelf item that is miniature in size and consumes less than 1watt of power.

The uniqueness of the TDRSS MA capability introduces many diverseapplications, to a variety of government agencies, that have not beenfeasible to date via compact, low-power field transceivers. Examplesinclude: global search and rescue; special operations support; periodicreceipt of science data from unmanned, remote science stations, globaleducational interconnectivity, telemedicine and global broadcasting. Inaddition, the TDRSS global, random-access capability may introduce newpossibilities for mobile communication concept/technology developmentand testing, and other developments and experimentation, all of whichmay support U.S. industry in enhancing its internationalcompetitiveness. What is especially significant here is that the TDRSSis an existing space asset, that is immediately available withoutmodification, and permits application of a real (rather than simulated)satellite communication channel.

It will be understood that the invention has been described in terms ofpreferred embodiments, and that modifications and adaptations may bemade therein without department from the true scope and spirit of theinvention as defined by the following claims.

What is claimed is:
 1. In a tracking and data relay satellite systemhaving a satellite communication system that derives satellite antennapointing/nulling through a linear, weighted combination of multiplesignals at a ground terminal, and wherein the signals are derived froman array of broad-coverage antenna elements on one or more earthorbiting satellites having a space-to-ground link with a compositedownlink signal composed of the signals from the elements, theimprovement wherein the beams formed on the ground for the return linkand on the satellite for the forward link are structured so as to nullone or more regions within their field of view.
 2. In a tracking anddata relay satellite system having a satellite communication system thatderives satellite antenna pointing/nulling through a linear, weightedcombination of multiple signals at a ground terminal, and wherein thesignals are derived from an array of broad-coverage antenna elements onone or more earth orbiting satellites having a space-to-ground link witha composite downlink signal composed of the signals from the elements,the improvement for providing additional communications, comprisingbidirectional packet data transfer, using altered ground terminalreceiver and a related field transceiver, using waveforms, scheduling,beamforming to prevent interference with communications functions ofsaid satellite system, and using acquisition technology to permitreception of short duration packets.
 3. In a tracking and data relaysatellite system having a satellite communication system that derivessatellite antenna pointing/nulling through a linear, weightedcombination of multiple signals at a ground terminal, and wherein thesignals are derived from an array of broad-coverage antenna elements onone or more earth orbiting satellites having a space-to-ground link witha composite downlink signal composed of the signals from the elements,said satellite system having other communications resources includingsingle access S-, Ku- and future Ka-bands, the improvement for providingadditional communication, comprising means providing alternative oraugmented services which do not conflict with normal missioncommunications; conflict-free service augmentation by selectiveallocation of service resources and/or use of unscheduled service timewhen no normal mission communication is using the service; use of suchunscheduled service selectively through a store-and-forward approach inwhich the user or ground terminals collects and stores information untilsuch time as service is available; once service is available, theinformation is transmitted in whole or in part depending on the amountof service available; additional conflict-free service augmentationselectively through allocation of unused frequency on a single-accessantenna, for independent user in field-of-view.
 4. In a tracking anddata relay satellite system having a satellite communication system thatderives satellite antenna pointing/nulling through a linear, weightedcombination of multiple signals at a ground terminal, and wherein thesignals are derived from an array of broad-coverage antenna elements onone or more earth orbiting satellites having a space-to-ground link witha composite downlink signal composed of the signals from the elements,the improvement for providing additional communication, comprising aGlobal Broadcast System (GBS), implementation means for said GBSincluding high bandwidth selected from Ku- or Ka-band, and selectedS-band component services enable information broadcast from the groundterminals to the field user transceiver; a return link service means;and means enabling field user requests for information to be made viasaid return link service means.